Not Applicable.
Not Applicable.
1. Field of the Invention
A disclosed embodiment of the invention relates generally to the detection of errors in ultrasonic transit time measurements. Even more particularly, a disclosed embodiment of the invention relates to the identification of mistakes in peak selection for the ultrasonic waveforms.
2. Description of the Related Art
After a hydrocarbon such as natural gas has been removed from the ground, the gas stream is commonly transported from place to place via pipelines. As is appreciated by those of skill in the art, it is desirable to know with accuracy the amount of gas in the gas stream. Particular accuracy for gas flow measurements is demanded when gas (and any accompanying liquid) is changing hands, or xe2x80x9ccustody.xe2x80x9d Even where custody transfer is not taking place, however, measurement accuracy is desirable.
Gas flow meters have been developed to determine how much gas is flowing through the pipeline. An orifice meter is one established meter to measure the amount of gas flow. Certain drawbacks with this meter existed, however. More recently, another type of meter to measure gas flow was developed. This more recently developed meter is called an ultrasonic flow meter.
FIG. 1A shows an ultrasonic meter suitable for measuring gas flow. Spoolpiece 100, suitable for placement between sections of gas pipeline, has a predetermined size and thus defines a measurement section. Alternately, a meter may be designed to attach to a pipeline section by, for example, hot tapping. As used herein, the term xe2x80x9cpipelinexe2x80x9d when used in reference to an ultrasonic meter may be referring also to the spoolpiece or other appropriate housing across which ultrasonic signals are being sent. A pair of transducers 120 and 130, and their respective housings 125 and 135, are located along the length of spoolpiece 100. A path 110, sometimes referred to as a xe2x80x9cchordxe2x80x9d exists between transducers 120 and 130 at an angle xcex8 to a centerline 105. The position of transducers 120 and 130 may be defined by this angle, or may be defined by a first length L measured between transducers 120 and 130, a second length X corresponding to the axial distance between points 140 and 145, and a third length D corresponding to the pipe diameter. Distances D, X and L are precisely determined during meter fabrication. Points 140 and 145 define the locations where acoustic signals generated by transducers 120 and 130 enter and leave gas flowing through the spoolpiece 100 (i.e. the entrance to the spoolpiece bore). In most instances, meter transducers such as 120 and 130 are placed a specific distance from points 140 and 145, respectively, regardless of meter size (i.e. spoolpiece size). A fluid, typically natural gas, flows in a direction 150 with a velocity profile 152. Velocity vectors 153-158 indicate that the gas velocity through spool piece 100 increases as centerline 105 of spoolpiece 100 is approached.
Transducers 120 and 130 are ultrasonic transceivers, meaning that they both generate and receive ultrasonic signals. xe2x80x9cUltrasonicxe2x80x9d in this context refers to frequencies above about 20 kilohertz. Typically, these signals are generated and received by a piezoelectric element in each transducer. To generate an ultrasonic signal, the piezoelectric element is stimulated electrically, and it responds by vibrating. This vibration of the piezoelectric element generates an ultrasonic signal that travels across the spoolpiece to the corresponding transducer of the transducer pair. Similarly, upon being struck by an ultrasonic signal, the receiving piezoelectric element vibrates and generates an electrical signal that is detected, digitized, and analyzed by electronics associated with the meter.
Initially, D (xe2x80x9cdownstreamxe2x80x9d) transducer 120 generates an ultrasonic signal that is then received at, and detected by, U (xe2x80x9cupstreamxe2x80x9d) transducer 130. Some time later, U transducer 130 generates a return ultrasonic signal that is subsequently received at and detected by D transducer 120. Thus, U and D transducers 130 and 120 play xe2x80x9cpitch and catchxe2x80x9d with ultrasonic signals 115 along chordal path 110. During operation, this sequence may occur thousands of times per minute.
The transit time of the ultrasonic wave 115 between transducers U 130 and D 120 depends in part upon whether the ultrasonic signal 115 is traveling upstream or downstream with respect to the flowing gas. The transit time for an ultrasonic signal traveling downstream (i.e. in the same direction as the flow) is less than its transit time when traveling upstream (i.e. against the flow). In particular, the transit time t1, of an ultrasonic signal traveling against the fluid flow and the transit time t2 of an ultrasonic signal travelling with the fluid flow may be defined:                               t          1                =                  L                      c            -                          V              ⁢                              x                L                                                                        (        1        )                                          t          2                =                  L                      c            +                          V              ⁢                              x                L                                                                        (        2        )            
where,
c=speed of sound in the fluid flow;
V=average axial velocity of the fluid flow over the chordal path in the axial direction;
L=acoustic path length;
x=axial component of L within the meter bore;
t1=transmit time of the ultrasonic signal against the fluid flow; and
t2=transit time of the ultrasonic signal with the fluid flow.
The upstream and downstream transit times can be used to calculate the average velocity along the signal path by the equation:                     V        =                                                            L                2                                                              2                  ⁢                  x                                ⁢                                  xe2x80x83                                                      ⁢                          t              1                                -                                    t              2                                                      t                1                            ⁢                              t                2                                                                        (        3        )            
with the variables being defined as above.
The upstream and downstream travel times may also be used to calculate the speed of sound in the fluid flow according to the equation:                     c        =                                            L              2                        ⁢                          t              1                                +                                    t              2                                                      t                1                            ⁢                              t                2                                                                        (        4        )            
To a close approximation, equation (4) may be restated as:                     V        =                                            c              2                        ⁢            Δ            ⁢                          xe2x80x83                        ⁢            t                                2            ⁢            x                                              (        5        )            
where,
xcex94t=t1xe2x88x92t2xe2x80x83xe2x80x83(6)
So to a close approximation at low velocities, the velocity v is directly proportional to xcex94t.
Given the cross-section measurements of the meter carrying the gas, the average velocity over the area of the meter bore may be used to find the volume of gas flowing through the meter or pipeline 100.
In addition, ultrasonic gas flow meters can have one or more paths. Single-path meters typically include a pair of transducers that projects ultrasonic waves over a single path across the axis (i.e. center) of spoolpiece 100. In addition to the advantages provided by single-path ultrasonic meters, ultrasonic meters having more than one path have other advantages. These advantages make multi-path ultrasonic meters desirable for custody transfer applications where accuracy and reliability are crucial.
Referring now to FIG. 1B, a multi-path ultrasonic meter is shown. Spool piece 100 includes four chordal paths A, B, C, and D at varying levels through the gas flow. Each chordal path A-D corresponds to two transceivers behaving alternately as a transmitter and receiver. Also shown is an electronics module 160, which acquires and processes the data from the four chordal paths A-D. This arrangement is described in U.S. Pat. No. 4,646,575, the teachings of which are hereby incorporated by reference. Hidden from view in FIG. 1B are the four pairs of transducers that correspond to chordal paths A-D.
The precise arrangement of the four pairs of transducers may be more easily understood by reference to FIG. 1C. Four pairs of transducer ports are mounted on spool piece 100. Each of these pairs of transducer ports corresponds to a single chordal path of FIG. 1B. A first pair of transducer ports 125 and 135 includes transducers 120 and 130 recessed slightly from the spool piece 100. The transducers are mounted at a non-perpendicular angle xcex8 to centerline 105 of spool piece 100. Another pair of transducer ports 165 and 175 including associated transducers is mounted so that its chordal path loosely forms an xe2x80x9cXxe2x80x9d with respect to the chordal path of transducer ports 125 and 135. Similarly, transducer ports 185 and 195 are placed parallel to transducer ports 165 and 175 but at a different xe2x80x9clevelxe2x80x9d (i.e. a different radial position in the pipe or meter spoolpiece). Not explicitly shown in FIG. 1C is a fourth pair of transducers and transducer ports. Taking FIGS. 1B and 1C together, the pairs of transducers are arranged such that the upper two pairs of transducers corresponding to chords A and B form an X and the lower two pairs of transducers corresponding to chords C and D also form an X.
Referring now to FIG. 1B, the flow velocity of the gas may be determined at each chord A-D to obtain chordal flow velocities. To obtain an average flow velocity over the entire pipe, the chordal flow velocities are multiplied by a set of predetermined constants. Such constants are well known and were determined theoretically.
Because the measurement of gas flow velocity and speed of sound depend on measured transit time, t, it is important to measure transit time accurately. One method and apparatus for measuring the time of flight of a signal is disclosed in U.S. Pat. No. 5,983,730, issued Nov. 16, 1999, entitled xe2x80x9cMethod and Apparatus for Measuring the Time of Flight of A Signalxe2x80x9d, which is hereby incorporated by reference for all purposes.
A difficulty that arises in measuring a time of flight exactly is defining when an ultrasonic waveform is received. For example, a detected waveform corresponding to a received ultrasonic signal may look like that shown in FIG. 2. The precise instant this waveform is deemed to have arrived is not altogether clear. One method to define the arrival instant is to define it as a zero crossing that follows a predefined voltage threshold value for the waveform. However, signal degradation due to pressure fluctuations for example may cause the correct zero crossing to be misidentified, as shown in FIG. 3 (not to scale). Other methods for identifying arrival time may also be used, but each is also subject to measurement error by misidentification of the proper arrival time.
Further, regardless of the method of measuring arrival time, adaptive methods are known that modify the measurement criterion or criteria values during measurement of the ultrasonic signals. These methods were developed to be more accurate than other methods of measuring arrival time but nonetheless still suffer from misidentification of the ultrasonic signal arrival time, leading to measurement errors. Further, because of the adaptive nature of the identification method, the misidentification tends to be xe2x80x9clocked inxe2x80x9d, where the detection algorithm tends to lock in the mistake so that it happens time after time after time.
Although the misidentification of the arrival time of ultrasonic signals has long been known, previous approaches to identifying peak switching errors are inadequate. For example, ultrasonic meters generally provide a speed of sound reading for the gas flow through the meters. This speed of sound calculation varies with error in the peak selection. However, the variation in the speed of sound measurement is small and may be difficult to identify directly. In addition, the variation tends to get smaller as the meter size increases, making uniform diagnostics nearly impossible.
There therefore exists a need for a method or ultrasonic meter that eliminates misidentification of the arrival time of ultrasonic signals. Ideally, such a method or meter would be compatible with existing meters. It would also be ideal if such a method or meter were inexpensive to implement.
A method to detect a peak selection error in a waveform includes measuring a first average transit time for one or more ultrasonic signals along a first path in a meter, measuring a second average transit time for one or more ultrasonic signals along a second path in the meter (where the first and second paths are of different lengths) and determining whether there exists a peak selection error based on the length of the first path, the length of the second path, the first average transit time, and the second average transit time.